This invention relates to an improved roller cone coring bit used to cut a core of a subterranean formation. More particularly, the improved design includes the use of spiral stabilizer blades to stabilize the bit and thereby enhance the ability to cut and recover a continuous core of a reservoir rock.
Analysis of core samples yields important geological information about subterranean formations. There are two basic methods of obtaining a core sample. Coring may be done at the time of drilling or sidewall core samples may be taken after the hole has been drilled. Coring at the time of drilling utilizes some type of open center bit which cuts a donut-shaped hole, leaving a cylindrical plug or core in the center. As drilling progresses, the central plug or core rises inside a hollow tube or core barrel above the bit where it is captured and subsequently retrieved at the surface. Coring bits come in three basic varieties: diamond core heads, polycrystalline diamond core heads, or roller cone coring bits. This invention relates to an improved roller cone coring bit.
Roller cone coring bits tend to drill a slightly oversized borehole. The slightly oversized borehole creates an exaggerated rotation pattern for the core bit. Due to the relatively low tensile strength of most rock, the exaggerated rotation, or whirl, tends to break or shear the core. This is undesirable for several reasons. The inner core barrel may become jammed thereby preventing the recovery of additional core. Alternatively, only fragmented pieces of a core will be recovered which reduces the quality and quantity of information that can be obtained from the core.
With conventional roller cone coring bits, only the cone cutters extended to the gage diameter of the borehole. The bit body and the journal arm segments are smaller than the gage diameter of the borehole. To reduce the effects of the whirling motion, coring companies have attempted to stabilize the outer core barrel above the core bit. However, this did not eliminate the whirling action. Although relatively close to the bit, the stabilizer on the outer core barrel acted as a fulcrum point. This created a moment on the core bit, which increased the tendency to drill an oversized borehole.
To overcome the above problem, stabilizer pads were welded onto the body of the core bit. These pads tended to be square in shape and proved to be marginally successful. The stabilizer pads on the bit body tended to reduced the whirling motion and the fulcrum effect. However, laboratory testing showed that the straight vertical leading edge of the square pads would engage the borehole wall in such a manner to actually increase the whirling motion of the bit under certain conditions. When these conditions occur, the square stabilizer pads are actually detrimental to coring.
This invention overcomes the problems described above. The roller cone core bit of the present invention includes spiral stabilizer blades. The proximity of the stabilizer blades to the cutting elements, i.e., the cone cutters, reduces the tendency to drill an oversized hole. In addition, the spiral shaped stabilizer blades are less likely to hang up on the borehole wall than the prior art square stabilizer pads. As a result, a smoother drilling action is accomplished. The smoother drilling action enhances the ability to recover a continuous core.